With development of portable devices such as personal computers and mobile phones, nowadays, there is an increasing demand for batteries as power sources for these devices. Batteries used for these devices are used at room temperatures, and are required to have a high energy density as well as excellent cycle performance.
For such demands, a battery including simple substance of silicon (Si) or tin (Sn), an oxide of Si or Sn, or an alloy including Si or Sn as the negative electrode active material, which achieves a greatly high capacity, is considered as promising.
However, lithium absorption causes changes in the crystal structure of these materials, leading to a volume increase. The large extent of volume change in the active material upon charging and discharging causes a contact failure between the active material and the current collector, leading to a shortened charge and discharge cycle life.
A negative electrode deformation is also a problem when using such materials as the negative electrode active material. Upon charging and discharging, intercalation and deintercalation of lithium (Li) cause a large degree of expansion and contraction in the negative electrode active material, leading to a large degree of negative electrode distortion and undulating. Thus, gaps are created between the negative electrode and the separator, which causes nonuniform charging and discharging reactions and declines the cycle performance.
For such a problem, there has been proposed that a space is provided in the negative electrode for easing the stress from the active material expansion, to retard the distortion and undulating, and deterioration in cycle performance. For example, Japanese Laid-Open Patent Publication No. 2003-303586 has proposed that columnar particles of silicon are formed on the current collector. Additionally, Japanese Laid-Open Patent Publication No. 2004-127561 has proposed that an active material that forms an alloy with lithium is disposed orderly on the current collector.
In the negative electrode of Japanese Laid-Open Patent Publication No. 2003-303586 and the negative electrode of Japanese Laid-Open Patent Publication No. 2004-127561, columnar negative electrode active material particles that are grown in the direction parallel to the direction normal to the current collector are formed on the sheet-like current collector. Thus, a greater portion of the positive electrode active material layer faces the exposed portion of the negative electrode current collector, rather than facing the negative electrode active material. Therefore, lithium supplied from the positive electrode active material while charging is not absorbed by the negative electrode active material, and easily deposits on the exposed portion of the negative electrode current collector. As a result, while discharging, lithium is not released from the negative electrode effectively, causing a decline in charge and discharge efficiency.
Also, while discharging, because only the portion of the positive electrode active material layer facing the negative electrode active material reacts easily, in addition to the decline in actual discharge capacity, there is a possibility that the positive electrode active material becomes partially over-discharged. Repetitive charge and discharge cycles advance such non-uniform reaction, and the portion of the positive electrode active material that was not discharged easily becomes over-charged. On the other hand, the portion of the positive electrode active material that is in over-discharged state also increases. Thus, the side reaction increases its proportion, the discharge capacity declines, and battery deterioration advances. Especially when charge and discharge are carried out at a high current, the cycle performance drastically declines.
Further, when the columnar particles expand and the adjacent columnar particles contact with each other, a stress is applied to the interface between the columnar particles and the current collector, causing a breakage in the columnar particles, and a separation of the active material from the current collector. This leads to a decrease in current collecting ability and in cycle performance.